arXiv subjects

G. Nelemans

Publications and source records attributed to G. Nelemans.

At least 73 records · Page 4Linked to original sources

arXiv2012

On the point mass approximation to calculate the gravitational wave signal from white dwarf binaries

Double white dwarf binaries in the Galaxy dominate the gravitational wave sky and would be detectable for an instrument such as LISA. Most studies have calculated the expected gravitational wave signal under the assumption that the binary white dwarf system can be represented by two point masses in orbit. We discuss the accuracy of this approximation for real astrophysical systems. For non-relativistic binaries in circular orbit the gravitational wave signal can easily be calculated. We show that for these systems the point mass approximation is completely justified when the individual stars are axisymmetric irrespective of their size. We find that the signal obtained from Smoothed-Particle Hydrodynamics simulations of tidally deformed, Roche-lobe filling white dwarfs, including one case when an accretion disc is present, is consistent with the point mass approximation. The difference is typically at the level of one per cent or less in realistic cases, yielding small errors in the inferred parameters of the binaries.

astro-ph.HE↗

arXiv2012

Formation of the planet around the millisecond pulsar J1719-1438

Context. Recently the discovery of PSR J1719-1438, a 5.8 ms pulsar with a companion in a 2.2 hr orbit, was reported. The combination of this orbital period and the very low mass function is unique. The discoverers, Bailes et al., proposed an ultracompact X-ray binary (UCXB) as the progenitor system. However, the standard UCXB scenario would not produce this system as the time required to reach this orbital period exceeds the current estimate of the age of the Universe. The detached state of the system aggravates the problem. Aims. We want to understand the evolutionary history of PSR J1719-1438, and determine under which circumstances it could have evolved from an UCXB. Methods. We model UCXB evolution varying the donor size and investigate the effect of a wind mass loss from the donor, and compare the results with the observed characteristics of PSR J1719-1438. Results. An UCXB can reach a 2.2 hr orbit within the age of the Universe, provided that 1) the millisecond pulsar can significantly heat and expand the donor by pulsar irradiation, or 2) the system loses extra orbital angular momentum, e.g. via a fast wind from the donor. Conclusions. The most likely scenario for the formation of PSR J1719-1438 is UCXB evolution driven by angular momentum loss via the usual gravitational wave emission, which is enhanced by angular momentum loss via a donor wind of ~3x10^-13 Msun/yr. Depending on the size of the donor during the evolution, the companion presently probably has a mass of ~1-3 Jupiter masses, making it a very low mass white dwarf as proposed by Bailes et al. Its composition can be either helium or carbon-oxygen. A helium white dwarf companion makes the long (for an UCXB) orbital period easier to explain, but the required inclination makes it a priori less likely than a carbon-oxygen white dwarf.

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arXiv2012

Discovery of a stripped red giant core in a bright eclipsing binary star

We report the serendipitous discovery from WASP archive photometry of a binary star in which an apparently normal A-type star (J0247-25A) eclipses a smaller, hotter subdwarf star (J0247-25B). The kinematics of J0247-25A show that it is a blue-straggler member of the Galactic thick-disk. We present follow-up photometry and spectroscopy from which we derive approximate values for the mass, radius and luminosity for J0247-25B assuming that J0247-25A has the mass appropriate for a normal thick-disk star. We find that the properties of J0247-25B are well matched by models for a red giant stripped of its outer layers and currently in a shell hydrogen-burning stage. In this scenario, J0247-25B will go on to become a low mass white dwarf (M~0.25 solar masses) composed mostly of helium. J0247-25B can be studied in much greater detail than the handful of pre helium white dwarfs (pre-He-WD) identified to-date. These results have been published by Maxted et al., 2011. We also present a preliminary analysis of more recent observations of J0247-25 with the UVES spectrograph, from which we derive much improved masses for both stars in the binary. We find that both stars are more massive than expected and that J0247-25A rotates sub-synchronously by a factor of about 2. We also present lightcurves for 5 new eclipsing pre-He-WD subsequently identified from the WASP archive photometry, 4 of which have mass estimates for the subdwarf companion based on a pair of radial velocity measurements.

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arXiv2011

Type Ia supernovae in globular clusters: observational upper limits

In the dense stellar environment of the globular clusters, compact binaries are produced dynamically. Therefore the fraction of type Ia supernovae that explode in globular clusters is expected to be higher than the fraction of mass residing in these. We have searched for globular clusters at the positions of observed type Ia supernovae. We used archival HST images and literature data, covering the positions either before the supernovae exploded, or long enough after that the supernovae have faded below the luminosities of globular clusters. We did not find evidence for globular clusters at any of the supernova positions. For 18 type Ia supernovae, the observations are sensitive enough that any globular cluster would have been detected, and for further 17 type Ia supernovae, the brighter globular clusters would have been detected. Correcting for incompleteness, we derive a 90% upper limit of 0.09 on the fraction of type Ia supernovae that explode in globular clusters for the full sample and 0.22 for the sample of supernovae in late-type galaxies. This allows us to limit enhancements per unit stellar mass for a coeval population eta_{co}<50 (100) with 90% (99%) confidence. We find that by observing the positions of a sample of less than 100 type Ia supernovae in the outer parts of early-type galaxies, it will be possible to probe the currently favoured range of eta_{co}~1-10.

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arXiv2011

The evolution of ultracompact X-ray binaries

Context. Ultracompact X-ray binaries (UCXBs) typically consist of a white dwarf donor and a neutron star or black hole accretor. The evolution of UCXBs and very low mass ratio binaries in general is poorly understood. Aims. We investigate the evolution of UCXBs in order to learn for which mass ratios and accretor types these systems can exist, and if they do, what are their orbital and neutron star spin periods, mass transfer rates and evolutionary timescales. Methods. For different assumptions concerning accretion disk behavior we calculate for which system parameters dynamical instability, thermal-viscous disk instability or the propeller effect emerge. Results. At the onset of mass transfer, the survival of the UCXB is determined by how efficiently the accretor can eject matter in the case of a super-Eddington mass transfer rate. At later times, the evolution of systems strongly depends on the binary's capacity to return angular momentum from the disk to the orbit. We find that this feedback mechanism most likely remains effective. In the case of steady mass transfer, the propeller effect can stop accretion onto recycled neutron stars completely at a sufficiently low mass transfer rate, based on energy considerations. However, mass transfer will likely be non-steady because disk instability allows for accretion of some of the transferred matter. Together, the propeller effect and disk instability cause the low mass ratio UCXBs to be visible a small fraction of the time at most, thereby explaining the lack of observations of such systems. Conclusions. Most likely UCXBs avoid late-time dynamically unstable mass transfer and continue to evolve as the age of the Universe allows. This implies the existence of a large population of low mass ratio binaries with orbital periods ~ 70 - 80 min, unless some other mechanism has destroyed these binaries.

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arXiv2011

PG1018-047: the longest period subdwarf B binary

About 50% of all known hot subdwarf B stars (sdBs) reside in close (short period) binaries, for which common envelope ejection is the most likely formation mechanism. However, Han et al. 2003 predict that the majority of sdBs should form through stable mass transfer leading to long period binaries. Determining orbital periods for these systems is challenging and while the orbital periods of ~100 short period systems have been measured, there are no periods measured above 30 days. As part of a large program to characterise the orbital periods of subdwarf B binaries and their formation history, we have found that PG1018-047 has an orbital period of 760\pm6 days, easily making it the longest period ever detected for a subdwarf B binary. Exploiting the Balmer lines of the subdwarf primary and the narrow absorption lines of the companion present in the spectra, we derive the radial velocity amplitudes of both stars, and estimate the mass ratio M_{MS}/M_{sdB} = 1.6\pm0.2. From the combination of visual and infrared photometry, the spectral type of the companion star is determined to be mid K.

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arXiv2011

On the association of ULXs with young superclusters: M82 X-1 and a new candidate in NGC 7479

We investigate the spatial coincidence of ultra-luminous X-ray sources (ULXs) with young massive stellar clusters. In particular we perform astrometry on Chandra and HST data of two ULXs that are possibly associated with such clusters. To date M82 X-1 is the only ULX claimed to be coincident with a young massive stellar cluster. We remeasure the position of this source with a high accuracy and find that the position of the X-ray source is 0.65 arcsec away from the stellar cluster, corresponding to an offset significance of 3 sigma. We also report the discovery of a new candidate, based on observations of NGC 7479. One of the ULXs observed in three X-ray observations is found to be spatially coincident (within 1 sigma of the position error) with a young super-cluster observed in the HST images. In the brightest state, the absorbed luminosity of the ULX is a few times $10^{40}$ erg s$^{-1}$, and in the faintest state below the detection limit of $\sim4$ times $10^{39}$ erg s$^{-1}$. The luminosity in the brightest state requires an accreting black hole mass of at least 100 M$_{\odot}$ assuming isotropic emission. However it is possible that the source is contaminated by X-ray emission from the nearby supernova SN2009jf. In this case the luminosity of the ULX is in a range where it is strongly debated whether it is a super-Eddington stellar mass black hole or an intermediate mass black hole. The colours of the host cluster indicate a young stellar population, with an age between 10 and 100 Myr. The total stellar mass of the cluster is $\sim5\cdot10^{5}$M$_{\odot}$.

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arXiv2011

Discovery of a stripped red giant core in a bright eclipsing binary system

We have identified a star in the WASP archive photometry with an unusual lightcurve due to the total eclipse of a small, hot star by an apparently normal A-type star and with an orbital period of only 0.668d. From an analysis of the WASP lightcurve together with V-band and I_C-band photometry of the eclipse and a spectroscopic orbit for the A-type star we estimate that the companion star has a mass of (0.23+-0.03)Msun and a radius of (0.33+-0.01)Rsun, assuming that the A-type star is a main-sequence star with the metalicity appropriate for a thick-disk star. The effective temperature of the companion is (13400+-1200)K from which we infer a luminosity of (3+-1)Lsun. From a comparison of these parameters to various models we conclude that the companion is most likely to be the remnant of a red giant star that has been very recently stripped of its outer layers by mass transfer onto the A-type star. In this scenario, the companion is currently in a shell hydrogen-burning phase of its evolution, evolving at nearly constant luminosity to hotter effective temperatures prior to ceasing hydrogen burning and fading to become a low-mass white dwarf composed of helium (He-WD). The system will then resemble the pre-He-WD/He-WD companions to A-type and B-type stars recently identified from their Kepler satellite lightcurves (KOI-74, KOI-81 and KIC10657664). This newly discovered binary offers the opportunity to study the evolution of a stripped red giant star through the pre-He-WD stage in great detail.

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arXiv2011

A deep radio survey of hard state and quiescent black hole X-ray binaries

We have conducted a deep radio survey of a sample of black hole X-ray binaries in the hard and quiescent states, to determine whether any systems were sufficiently bright for astrometric follow-up with high-sensitivity very long baseline interferometric (VLBI) arrays. The one hard-state system, Swift J1753.5-0127, was detected at a level of 0.5 mJy/beam. All eleven quiescent systems were not detected. In the three cases with the highest predicted quiescent radio brightnesses (GRO J0422+32, XTE J1118+480, and GRO J1655-40), the new capabilities of the Expanded Very Large Array were used to reach noise levels as low as 2.6 microJy/beam. None of the three sources were detected, to 3-sigma upper limits of 8.3, 7.8, and 14.2 microJy/beam, respectively. These observations represent the most stringent constraints to date on quiescent radio emission from black hole X-ray binaries. The uncertainties in the source distances, quiescent X-ray luminosities at the times of the observations, and in the power-law index of the empirical correlation between radio and X-ray luminosities, make it impossible to determine whether these three sources are significantly less luminous in the radio band than expected. Thus it is not clear whether that correlation holds all the way down to quiescence for all black hole X-ray binaries.

astro-ph.HE↗

arXiv2011

The Galactic Bulge Survey: outline and X-ray observations

We introduce the Galactic Bulge Survey (GBS) and we provide the Chandra source list for the region that has been observed to date. Among the goals of the GBS are constraining the neutron star equation of state and the black hole mass distribution via the identification of eclipsing neutron star and black hole low-mass X-ray binaries. The latter goal will, in addition, be obtained by significantly enlarging the number of black hole systems for which a black hole mass can be derived. Further goals include constraining X-ray binary formation scenarios, in particular the common envelope phase and the occurrence of kicks, via source-type number counts and an investigation of the spatial distribution of X-ray binaries, respectively. The GBS targets two strips of 6x1 degrees (12 square degrees in total), one above (1<b<2 degrees) and one below (-2<b<-1 degrees) the Galactic plane in the direction of the Galactic Center at both X-ray and optical wavelengths. By avoiding the Galactic plane (-1<b<1 degrees) we limit the influence of extinction on the X-ray and optical emission but still sample relatively large number densities of sources. The survey is designed such that a large fraction of the X-ray sources can be identified from their optical spectra. The X-ray survey, by design, covers a large area on the sky while the depth is shallow using 2 ks per Chandra pointing. In this way we maximize the predicted number ratio of (quiescent) low-mass X-ray binaries to Cataclysmic Variables. The survey is approximately homogeneous in depth to an 0.5-10 keV flux of 7.7x10^-14 erg cm-2 s-1. So far, we have covered about two-thirds (8.3 square degrees) of the projected survey area with Chandra providing over 1200 unique X-ray sources. We discuss the characteristics and the variability of the brightest of these sources.

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arXiv2011

Binaries discovered by the SPY survey VI. Discovery of a low mass companion to the hot subluminous planetary nebula central star EGB\,5 - A recently ejected common envelope?

Hot subdwarf B stars (sdBs) in close binary systems are assumed to be formed via common envelope ejection. According to theoretical models, the amount of energy and angular momentum deposited in the common envelope scales with the mass of the companion. That low mass companions near or below the core hydrogen-burning limit are able to trigger the ejection of this envelope is well known. The currently known systems have very short periods $\simeq0.1-0.3\,{\rm d}$. Here we report the discovery of a low mass companion ($M_{\rm 2}>0.14\,M_{\rm \odot}$) orbiting the sdB star and central star of a planetary nebula EGB\,5 with an orbital period of $16.5\,{\rm d}$ at a minimum separation of $23\,R_{\rm \odot}$. Its long period is only just consistent with the energy balance prescription of the common envelope. The marked difference between the short and long period systems will provide strong constraints on the common envelope phase, in particular if the masses of the sdB stars can be measured accurately. Due to selection effects, the fraction of sdBs with low mass companions and similar or longer periods may be quite high. Low mass stellar and substellar companions may therefore play a significant role for the still unclear formation of hot subdwarf stars. Furthermore, the nebula around EGB\,5 may be the remnant of the ejected common envelope making this binary a unique system to study this short und poorly understood phase of binary evolution.

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arXiv2011

The X-ray quiescence of Swift J195509.6+261406 (GRB 070610): an optical bursting X-ray binary?

We report on a 63ks Chandra observation of the X-ray transient Swift J195509.6+261406 discovered as the afterglow of what was first believed to be a long duration Gamma-Ray Burst (GRB 070610). The outburst of this source was characterized by unique optical flares on timescales of second or less, morphologically similar to the short X-ray bursts usually observed from magnetars. Our Chandra observation was performed ~2 years after the discovery of the optical and X-ray flaring activity of this source, catching it in its quiescent state. We derive stringent upper limits on the quiescent emission of Swif J195509.6+261406 which argues against the possibility of this object being a typical magnetar. Our limits show that the most viable interpretation on the nature of this peculiar bursting source, is a binary system hosting a black hole or a neutron star with a low mass companion star (< 0.12 M_{\odot}), and with an orbital period smaller than a few hours.

astro-ph.GA↗

arXiv2010

The binary companion of PSR J1740-3052

We report on the identification of a near-infrared counterpart to the massive (>11 Msun) binary companion of pulsar J1740-3052. An accurate celestial position of PSR J1740-3052 is determined from interferometric radio observations. Adaptive optics corrected near-infrared imaging observations show a counterpart at the interferometric position of the pulsar. The counterpart has Ks=15.87+-0.10 and J-Ks>0.83. Based on distance and absorption estimates from models of the Galactic electron and dust distributions these observed magnitudes are consistent with those of a main-sequence star as the binary companion. We argue that this counterpart is the binary companion to PSR J1740-3052 and thus rule out a stellar mass black hole as the pulsar companion.

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arXiv2010

SDSS J0926+3624: The shortest period eclipsing binary star

With orbital periods of the order of tens of minutes or less, the AM Canum Venaticorum stars are ultracompact, hydrogen deficient binaries with the shortest periods of any binary subclass, and are expected to be among the strongest gravitational wave sources in the sky. To date, the only known eclipsing source of this type is the P = 28 min binary SDSS J0926+3624. We present multiband, high time resolution light curves of this system, collected with WHT/ULTRACAM in 2006 and 2009. We supplement these data with additional observations made with LT/RISE, XMM_Newton and the Catalina Real-Time Transient Survey. From light curve models we determine the mass ratio to be q = M2 / M1 = 0.041 +/- 0.002 and the inclination to be 82.6 +/- 0.3 deg. We calculate the mass of the primary white dwarf to be 0.85 +/- 0.04 solar masses and the donor to be 0.035 +/- 0.003 solar masses, implying a partially degenerate state for this component. We observe superhump variations that are characteristic of an elliptical, precessing accretion disc. Our determination of the superhump period excess is in agreement with the established relationship between this parameter and the mass ratio, and is the most precise calibration of this relationship at low q. We also observe a quasi-periodic oscillation in the 2006 data, and we examine the outbursting behaviour of the system over a 4.5 year period.

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arXiv2010

A faint type of supernova from a white dwarf with a helium-rich companion

Supernovae (SNe) are thought to arise from two different physical processes. The cores of massive, short-lived stars undergo gravitational core collapse and typically eject a few solar masses during their explosion. These are thought to appear as as type Ib/c and II SNe, and are associated with young stellar populations. A type Ia SN is thought to arise from the thermonuclear detonation of a white dwarf star composed mainly of carbon and oxygen, whose mass approaches the Chandrasekhar limit. Such SNe are observed in both young and old stellar environments. Here we report our discovery of the faint type Ib SN 2005E in the halo of the nearby isolated galaxy, NGC 1032. The lack of any trace of recent star formation near the SN location (Fig. 1), and the very low derived ejected mass (~0.3 M_sun), argue strongly against a core-collapse origin for this event. Spectroscopic observations and the derived nucleosynthetic output show that the SN ejecta have high velocities and are dominated by helium-burning products, indicating that SN 2005E was neither a subluminous nor a regular SN Ia (Fig. 2). We have therefore found a new type of stellar explosion, arising from a low-mass, old stellar system, likely involving a binary with a primary white dwarf and a helium-rich secondary. The SN ejecta contain more calcium than observed in any known type of SN and likely additional large amounts of radioactive 44Ti. Such SNe may thus help resolve fundamental physical puzzles, extending from the composition of the primitive solar system and that of the oldest stars, to the Galactic production of positrons.

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arXiv2010

The Double-Peaked 2008 Outburst of the Accreting Milli-Second X-ray Pulsar, IGR J00291+5934

In August 2008, the accreting milli-second X-ray pulsar (AMXP), IGR J00291+5934, underwent an outburst lasting ~ 100 days, the first since its discovery in 2004. We present data from the double-peaked outburst from Faulkes Telescope North, the INT, the Keck Telescope, PAIRITEL, the Westerbork Synthesis Radio Telescope and the Swift, XMM-Newton and RXTE X-ray missions. We study the outburst's evolution at various wavelengths. We study the light curve morphology, presenting the first radio-X-ray Spectral Energy Distributions (SEDs) for this source and the most detailed UV-IR SEDs for any outbursting AMXP. We show simple models that attempt to identify the emission mechanisms responsible. We analyse short-timescale optical variability, and compare a medium resolution optical spectrum with those from 2004. The outburst morphology is unusual for an AMXP, comprising two peaks, the second containing a 'plateau' of ~ 10 days at maximum brightness within 30 days of the initial activity. This has implications on duty cycles of short-period X-ray transients. The X-ray spectrum can be fitted by a single, hard power-law. We detect optical variability of ~ 0.05 magnitudes, on timescales of minutes, but find no periodic modulation. In the optical, the SEDs contain a blue component, indicative of an irradiated disc, and a transient near-infrared (NIR) excess. This excess is consistent with a simple model of an optically thick synchrotron jet (as seen in other outbursting AMXPs). The optical spectrum shows a double-peaked H alpha profile, a diagnostic of an accretion disc, but we do not clearly see other lines (e.g. He I, II) reported in 2004. Optical/IR observations of AMXPs are excellent for studying the evolution of both the outer accretion disc and the inner jet, and may eventually provide us with tight constraints to model disc-jet coupling in accreting neutron stars.

astro-ph.HE↗

arXiv2010

The type Ib supernova 2010O: an explosion in a Wolf-Rayet X-ray binary?

The type Ib supernova 2010O was recently discovered in the interacting starburst galaxy Arp 299. We present an analysis of two archival Chandra X-ray observations of Arp 299, taken before the explosion and show that there is a transient X-ray source at a position consistent with the supernova. Due to the diffuse emission, the background is difficult to estimate. We estimate the flux of the transient from the difference of the two X-ray images and conclude that the transient can be described by a 0.225 keV black body with a luminosity of 2.5+/-0.7 10^{39} erg/s for a distance of 41 Mpc. These properties put the transient in between the Galactic black hole binary XTE J1550-564 and the ultra-luminous X-ray binaries NGC 1313 X-1 and X-2. The high level of X-ray variability associated with the active starburst makes it impossible to rule out a chance alignment. If the source is associated with the supernova, it suggests SN2010O is the explosion of the second star in a Wolf-Rayet X-ray binary, such as Cyg X-3, IC 10 X-1 and NGC 300 X-1.

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arXiv2010

Spectroscopic Evidence for a 5.4-Minute Orbital Period in HM Cancri

HM Cancri is a candidate ultracompact binary white dwarf with an apparent orbital period of only 5.4 minutes, as suggested by X-ray and optical light-curve modulations on that period, and by the absence of longer-period variability. In this Letter we present Keck-I spectroscopy which shows clear modulation of the helium emission lines in both radial velocity and amplitude on the 5.4-minute period and no other. The data strongly suggest that the binary is emitting He I 4471 from the irradiated face of the cooler, less massive star, and He II 4686 from a ring around the more massive star. From their relative radial velocities, we measure a mass ratio q=0.50+/-0.13. We conclude that the observed 5.4-minute period almost certainly represents the orbital period of an interacting binary white dwarf. We thus confirm that HM Cnc is the shortest-period binary star known: a unique test for stellar evolution theory, and one of the strongest known sources of gravitational waves for the Laser Interferometer Space Antenna (LISA).

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